Bone preparation instruments and methods

ABSTRACT

Instruments and methods for preparing adjacent bones for fusion, and for inserting implants are disclosed. In one embodiment, the instruments include paddles for spacing the adjacent bones a predetermined distance and a cutting edge to create a channel between the adjacent bones to receive a fusion implant. In another embodiment, the instruments include a bone cutting instrument, a rasp, and an implant inserting tool. The instruments and methods are particularly advantageous for preparing a spinal fusion implant site.

The present application is a continuation-in-part of Ser. No. 09/611,237filed Jul. 6, 2000.

FIELD OF THE INVENTION

This invention pertains to bone surgery. Specifically, the invention isdirected to instrumentation and methods for preparing adjacent bones forreceiving an implant therebetween. The invention is particularlyadvantageous for preparing an implant site for fusing vertebral bodiesto facilitate fusion.

BACKGROUND OF THE INVENTION

Chronic back problems can cause pain and disability for a large segmentof the population. Frequently, the cause of back pain is traceable todiseased disc material between opposing vertebrae. When the discmaterial is diseased, the opposing vertebrae may be inadequatelysupported, resulting in persistent pain.

Surgical techniques have been developed to remove the diseased discmaterial and fuse the joint between opposing vertebral bodies.Arthrodesis or fusion of the intervertebral joint can reduce the painassociated with movement of an intervertebral joint having diseased discmaterial. Generally, fusion techniques involve removal of the diseaseddisc and inserting a bone or non-bone implant between the opposingvertebral bodies to be fused.

Spinal fusion implants and related surgical instruments for implanting afusion device are known and disclosed in, for example, U.S. Pat. Nos.5,741,253; 5,722,977; 5,658,337; 5,609,636; 5,505,732; 5,489,308;5,489,307; 5,458,638; 5,055,104; 5,026,373; 5,015,247; 4,961,740;4,878,915; 4,834,757; 4,743,256; 4,501,269; and 3,848,601. Thedisclosure of each of these patents is incorporated herein by reference.

Often times, the degenerative changes of the diseased disc cause acollapse of the intervertebral disc space. Thus, prior to implantation,the intervertebral disc space may be distracted to restore the normalheight of the disc space or the normal relationship between thevertebrae to be fused. Maintaining the restored disc space height and/orvertebral relationships throughout preparation of the implant site canbe important for the ultimate stability at the fusion site.

The ease of use and efficiency of instruments and procedures used toprepare and place an implant at a fusion site is also very important tothe overall success of the procedure. For example, in addition to otherproblems, removal of unequal amounts of bone on either side of thefusion site, particularly in vertebral fusion procedures, can result inover reaming of one vertebra relative to the adjacent vertebra andultimately lead to a poorer surgical outcome.

Accordingly, there is a continuing need for instrumentation and methodsthat ensure precise placement of the implant as well as increasing theease and efficiency of the implant procedure. The present invention isdirected to this need.

SUMMARY OF THE INVENTION

The present invention is directed to bone cutting instruments andmethods that provide efficient and precise preparation of a bore forreceiving an implant between adjacent bones that are to be fused.

Throughout the specification, guidance may be provided through lists ofexamples. In each instance, the recited list serves only as arepresentative group. It is not meant, however, that the list isexclusive.

In one embodiment, the instruments of the invention include bone cuttinginstruments having paddles that can be inserted between adjacent bonesto maintain a fixed spacing between the bones during preparation of thebones for fusion. In one embodiment, the bone cutting instrumentsinclude a cutting edge that is fixedly mounted to the spacing paddles.In an alternative embodiment, the paddles can be mounted to a channelguide that provides a track for slidably positioning the cutting edge atthe site of bone preparation.

In a typical embodiment, a bone cutting instrument includes a cuttingedge that extends beyond the height dimensions of the paddles with aportion of the cutting edge extending between the paddles. Depending onthe configuration of the implant to be inserted between bones, thecutting edge can be circular, oval, rectangular, etc.

In another embodiment, a bone cutting instrument includes first, second,third, and fourth cutting edges that define an interior void, with thefirst and second cutting edges being opposite and extending beyond thethird and fourth cutting edges.

In a further embodiment, the invention involves a rasp adapted to definea recess between two bone surfaces. The rasp includes a shaft and a rasphead with an arcuate distal surface. At least one of the transversesurfaces of the rasp is roughened. Examples of roughened surfacesinclude knurls, etchings, ridges, grooves, teeth, etc.

In a still further embodiment, the invention involves an implantinsertion tool having a shaft with a pair of arms movable between aspaced apart holding position and a close together releasing position.The distal ends of the arms are shaped to fit inside an implant. Theinsertion tool also involves a sleeve operable on the arms to force thearms together towards the releasing position. The sleeve is hollow andis slidably mounted on the shaft. The sleeve forces the arms together torelease the implant when the sleeve is in the engaging position. In analternative embodiment, the sleeve is internally threaded and the shaftis externally threaded, with the threads matching. In this embodiment,the sleeve is rotated to move it along the shaft. The invention alsoprovides kits comprising one or more instruments of the invention havingvarious paddle and/or cutting edge heights, widths or shapes forpreparing an implant site of a predetermined size or shape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of one embodiment of a bone cutting instrumentaccording to the invention.

FIG. 2 is a close-up perspective view of the distal end of the bonecutting instrument of FIG. 1.

FIG. 3 is a distal end-on view of the bone cutting instrument of FIG. 1.

FIG. 4 is a perspective view of one embodiment of a non-bone implantsuitable for use according to the invention.

FIG. 5 is a perspective of an alternative embodiment of a bone cuttinginstrument according to the invention, including a channel guide andfirst and second mandrels.

FIG. 6 is a view of the same bone cutting instrument of FIG. 5 with thefirst and second mandrels removed and a bone chisel in the place of thefirst mandrel.

FIG. 7 is a top plan view of the channel guide of FIG. 5 (the oppositeside being substantially identical).

FIG. 8 is a side view of the channel guide of FIG. 7 (the opposite sideview being substantially identical).

FIG. 9 is a distal end view of the channel guide of FIG. 7.

FIG. 10 is a proximal end-on view of the channel guide of FIG. 7.

FIG. 11 is a top plan view of an alternative embodiment of a channelguide according to the invention.

FIG. 12 is a side plan view of the channel guide of FIG. 11.

FIG. 13 is a distal end-on view of the channel guide of FIG. 11.

FIG. 14 is a proximal end-on view of the channel guide of FIG. 11.

FIG. 15 is a top plan view of one embodiment of a mandrel according tothe invention.

FIG. 16 is a side plan view of the mandrel of FIG. 15.

FIG. 17 is a transverse cross-section view of the mandrel of FIG. 15through line 16—16.

FIG. 18 is a top plan view of one embodiment of a bone chisel accordingto the invention.

FIG. 19 is a longitudinal cross-section view taken through line 18—18 ofthe bone chisel of FIG. 18.

FIG. 20 is a distal end view of the bone chisel of FIG. 18.

FIG. 21 is a diagrammatical illustration of adjacent vertebrae havingchannels for receiving implants and prepared according to the invention.

FIG. 22 is a top view of another embodiment of a bone cutting instrumentaccording to the invention.

FIG. 23 is a side view of the bone cutting instrument of FIG. 22.

FIG. 24 is a distal end-on view of the bone cutting instrument of FIG.22.

FIG. 25 is a top view of a rasp according to the invention.

FIG. 26 is a side view of the rasp of FIG. 25.

FIG. 27 is a proximal end-on view of the rasp of FIG. 25.

FIG. 28 is an enlarged partial perspective view of the teeth on the rasphead of the invention.

FIG. 29 is an enlarged partial top view of the rasp head of FIG. 25.

FIG. 30 is a top view of an alignment pin of the invention.

FIG. 31 is a side view of the alignment pin of FIG. 30.

FIG. 32 is an exploded perspective view of the alignment pin of FIG. 30.

FIG. 33 is a top view of the handle of the alignment pin of theinvention.

FIG. 34 is a side view of the handle of FIG. 33.

FIG. 35 is a distal end-on view of the handle of FIG. 33.

FIG. 36 is a perspective view of a collar of the alignment pin of theinvention.

FIG. 37 is a top view of the collar of FIG. 36.

FIG. 38 is a side view of the collar of FIG. 36.

FIG. 39 is an end-on view of the collar of FIG. 36.

FIG. 40 is a top view of an implant insertion tool of the invention.

FIG. 41 is a side view of the implant insertion tool of FIG. 40.

FIG. 42 is a distal end-on view of the implant insertion tool of FIG.40.

FIG. 43 is a side view of a sleeve of the invention.

FIG. 44 is a side cross-sectional view of the sleeve of FIG. 43.

FIG. 45 is an end-on view of the sleeve of FIG. 43.

FIG. 46 is a top view of an insertion tool handle of the invention.

FIG. 47 is a cross-sectional view of the handle of FIG. 46.

FIG. 48 is a cross-sectional view of the handle of FIG. 46.

FIG. 49 is a perspective view of a two-part implant.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to instruments and methods forpreparing an implant site for receiving an implant between adjacentbones to be fused. The instruments of the invention can beadvantageously used for fusion of joints. In some embodiments, theinstruments and methods disclosed are particularly advantageous forpreparing an implant site for fusing cervical, thoracic and/or lumbarintervertebral joints. Thus, for exemplary purposes, the instruments andmethods of the invention will be described with reference to fusion of alumbar intervertebral joint. However, it will be appreciated that thedisclosed instruments and methods can be used for fusion of all types ofbones and particularly bones adjacent to a joint space. In the case offusing an intervertebral joint, the invention can be performed using ananterior, posterior or lateral approach to the patient's vertebrae.

As used herein, an “implant” includes any implant suitable forfacilitating fusion between adjacent bones and includes implantsprepared from known implant materials including, non-bone material suchas titanium, stainless steel, porous titanium, ceramic, etc. or boneincluding heterologous, homologous, autologous, artificial bone, etc.The implants suitable for the invention also can be threaded implants ornon-threaded.

An “implant site” refers to the location for placement of an implantbetween adjacent bones, such as adjacent vertebrae. In a typicalembodiment for vertebral fusion, the implant site can be a channelprepared by removing a notch from the opposing end plates of first andsecond vertebral bodies adjacent the intervertebral joint space.Preferably the notches are made through the articular cartilage andcortical bone into the cancellous bone. It will be appreciated that thenotches formed in the bone can be any shape suitable for receiving animplant of a particular shape including, for example, rectangular,circular, oval, etc. In the case of a circular channel, after formingthe channel, the channel can be threaded, using known tap systems, forreceiving a threaded implant.

Preparing an implant site according to the invention can be performedmore quickly and easily than prior procedures and can significantlyreduce surgery time and costs. Some cutting tools previously used toprepare implant sites are easy to use but lack certain precisioncharacteristics. For example, the distal end of some cutting tools maybe vulnerable to shifting from a desired location during cutting due toa lack of vertical stability, caused by, for example, irregularities orundulations at the surface of the vertebrae against which the distal endof the cutting tool is placed during cutting.

The disclosed devices can provide greater vertical stability and, in thecase of vertebral fusion, help to ensure that an equal amount of bone isremoved from the endplates of the vertebrae on either side of the jointspace. Removing equal amounts of bone can facilitate greater coaptationbetween the implant and the implant channel, greater fusion stability,greater motion segment stability, faster fusion, reduced pain, reducedchance of migration, reduced chance of subsidence, etc.

Throughout the specification, unless stated otherwise, the terms“proximal” and “distal” are relative terms, the term “proximal”referring to a location nearest the surgeon and the term “distal”referring to a location farthest from the surgeon. So, in the case ofperforming a vertebral fusion from an anterior approach, the anteriorsurfaces of the vertebrae are “proximal” and the posterior surfaces ofthe vertebrae are “distal” relative to the surgeon performing theprocedure. Likewise, in a posterior approach, the posterior vertebralsurfaces are proximal and the anterior surfaces are distal.

Generally, when preparing an implant site instruments used to preparethe site are advanced into the disc space from a proximal to distaldirection. That is, in an anterior approach the instruments are advancedfrom the anterior surface (proximal) towards the posterior surface(distal) and in a posterior approach the instruments are advanced fromthe posterior surface (proximal) towards the anterior surface (distal).Similar relative orientations also apply for lateral approaches.

As used herein, the “depth” of a vertebra is defined as the anteriorposterior dimension of the vertebrae. The “width” of the vertebrae isthe dimension from the right lateral edge to the left lateral edge. The“height” of the disc space is the dimension from the superior endplateto the inferior endplate of opposing vertebrae.

The implants can be sized for a particular application. For example, forstabilizing a lumbar disc space, the implant may have a height dimension“H” of about 2 mm to about 30 mm, a width dimension “W” of about 6 mm toabout 40 mm and a length dimension “L” of about 10 mm to about 40 mm.Other sizes will be appreciated as being within the scope of theinvention after review of the present disclosure. One instrument of theinvention, a bone cutting instrument or channel guide, has a proximalend and a distal end with a pair of paddles extending from the distalend of the instrument. In use, the paddles are placed into the spacebetween the bones to be fused to provide vertical stability of thedevice as well to maintain a selected spacing between the bones, whichis determined by the height of the paddles.

In general, the instruments will be available having varied paddleheights and varied widths between paddles. For cervical vertebralapplications a typical range of paddle heights can be approximately 2 mmto 12 mm, in 1 mm increments. For lumbar applications a typical range ofpaddle heights can be approximately 3 mm to 18 mm in 1 mm increments.However, larger or smaller widths with larger or smaller increments canbe available as needed. Thus, for example, in the case of vertebralfusion, a range of paddle heights will be available to establish andmaintain a selected joint space height between the vertebrae duringpreparation of the implant site.

Instruments having various widths or spacing between paddles will beavailable for different procedures. For example, if a single implant isto be used, it will typically have a greater width, and thus require apreparation instrument having a greater spacing between paddles, than ifmultiple implants will be used. A typical width between paddles for abone cutting instrument for placing a single implant can be about 4 mmto 40 mm. A typical width between paddles for a bone cutting instrumentfor implanting two implants between lumbar vertebrae will be about 4 mmto 24 mm.

The distal tip of the paddles can be tapered to facilitate insertion ofthe paddles into the joint space. In addition, the opposing edges of thepaddles can have a convergent or divergent taper from the distal tip toa proximal aspect of the paddle. A tapered paddle can provide a lordotictaper to the joint space to create a channel for receiving a taperedimplant for restoring or creating a particular degree of lordosisbetween the adjacent vertebral bodies.

A bone cutting instrument of the invention also includes a cutting edge.As will be further described below, the cutting edge can be separable ornon-separable from the paddles. In the case of a separable cutting edge,the instrument can include one or more tracks to guide the cutting edgeto a particular location. The cutting edge can be rectangular orcircular, oval, elliptical, oblong, etc. In one embodiment, the cuttingedge is a three-sided rectangle and provides for removing a rectangularnotch of bone.

The invention can be used with known starter guides, depth gauges, tapsand implant drivers used for preparing or inserting an implant into animplant site. Examples of suitable instruments are disclosed in U.S.Pat. Nos. 5,722,977; 5,658,337; 5,609,636; 5,489,307; 5,484,638;4,834,757; 3,848,601, etc., the entire disclosures of which areincorporated herein by reference.

In another embodiment, instruments for preparing a channel in adjacentbones and for inserting an implant are sized to be used with aparticular sized implant or implants. In this embodiment, each differentsize of implant has a corresponding set of bone cutting and implantinserting instruments. The instruments of the invention can be providedin kits including guides having paddles of different lengths and widthsand correspondingly sized bone cutting edges for spacing bones andpreparing implant sites for implants of various shapes and sizes. Inanother embodiment, the bone cutting instrument, rasp, and implantinsertion tool can be provided in a kit, with or without an implantsized and configured to be implanted using the instruments. In a furtherembodiment, a kit can be provided that includes a plurality ofincrementally sized implants and incrementally sized bone cuttinginstruments, rasps, and implant insertion tools so the user can selectthe appropriate size needed for a particular procedure.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

I. Instruments

The instruments and methods of the invention will now be described byreference to the accompanying drawings. The illustrated embodiments anddescription are provided only for exemplary purposes to facilitatecomprehension of the invention and should not be construed to limit thescope of the invention.

FIG. 1 is a side view and FIG. 2 an enlarged perspective view of thedistal end of one embodiment of a bone cutting instrument 10 accordingto the invention. Instrument 10 has a proximal end 15 and a distal end16 spaced along longitudinal axis X—X. At the proximal end 15 of shaft17 there is a handle 18 for operating instrument 10. At the distal end16, instrument 10 includes a first paddle 20, a second paddle 21 and acutting edge 23. In the illustrated embodiment, cutting edge 23 is atthe distal end of chamber 25. Proximal to cutting edge 23, chamber 25can include one or more openings 24 for passage of bone debris collectedwithin chamber 25 during cutting.

Paddles 20 and 21 include a tapered distal tip, 20 a and 21 a,respectively, to facilitate insertion of instrument 10 between adjacentbones. Proximal to the tapered distal ends 20 a and 21 a, instrument 10also includes markings 30 such as notches 31-34 at predetermineddistances from distal tips 20 a and 21 a. During use, markings 30provide the surgeon with an indication of the depth of distalpenetration of instrument 10 between adjacent bones. Furrows 36 and 37(not visible) are present along a portion of the sides 40 and 41,respectively, of paddles 20 and 21. Furrows 36 and 37 provide a reducedsurface area of paddle sides 38 and 39 and thus facilitate removal ofinstrument 10 from between adjacent bones.

FIG. 3 is a distal end-on view of instrument 10 showing that paddles 20and 21 each have the same height dimension P_(H) and a width dimensionW_(P) between paddles 20 and 21. A portion of cutting edge 23 is shownto extend beyond height dimension P_(H) at location 40 and 41 and aportion of cutting edge 23 is within the spacing between paddles 20 and21 at locations 42 and 43. The perimeter configuration of cutting edge23 in FIG. 3 is a parallelepiped shape particularly suited for preparinga channel or implant bore between adjacent bones for insertion of animplant having a cross-sectional configuration such as that of theimplant shown in FIG. 4. It will be appreciated, however, that theperimeter configuration of cutting edge 23 can be square, rectangular,circular, oval, etc., depending on the external configuration of theimplant to be inserted into the channel. In the illustrated embodiment,paddles 20 and 21 are fixedly attached to cutting edge 23. The paddlelength can vary to correspond with the depth of the vertebrae.

For any particular perimeter configuration, bone cutting instruments 10will be available which have incrementally varied sizes of cutting edge23 corresponding to a particular size implant. In addition, bone cuttinginstruments 10 having paddles with varied heights P_(H) will beavailable to permit the surgeon to select a paddle height correspondingto a particular disc space height. In addition, it will be appreciatedthat the illustrated paddle edges 44 a, 44 b (and 45 a, 45 b) areparallel. In alternative embodiments, edges 44 a, 44 b (and 45 a, 45 b)can form a converging or diverging taper.

FIG. 5 is a perspective view of an alternative embodiment of a bonecutting instrument 100. According to this embodiment, bone cuttinginstrument 100 has a proximal end 101, a distal end 102 and includes achannel guide 103, first mandrel 104 slidably received within a firsttrack 112 and a second mandrel 105 slidably received within a secondtrack 113. FIG. 6 illustrates bone cutting instrument 100 with first andsecond mandrels 104, 105 removed from tracks 112 and 113 of channelguide 103, and a bone chisel 106 slidably passed into track 112.

FIG. 7 is a top plan view of the channel guide 103, FIG. 8 is a sideview, FIG. 9 is a distal end-on view and FIG. 10 is a proximal end-onview. Channel guide 103 includes a distal end 110, a proximal end 111and a first track 112 and a second track 113 extending from the proximalend 111 to the distal end 110. Track 112 includes a base 112 a, a firstrail 112 b and a second rail 112 c. Track 113 includes a base 113 a, afirst rail 113 b and a second rail 113 c. In the illustrated embodiment,base 112 a of track 112 and base 113 a of track 113 are on opposingsurfaces of rail spacing member 114.

Extending distally from distal end 110, channel guide 103 includes afirst paddle 120 and a second paddle 121. Paddle 120 has a first edge120 a, a second edge 120 b and a tapered distal end 120 c. Likewise,paddle 121 has a first edge 121 a, a second edge 121 b and a tapereddistal end 121 c. Paddle spacing member 115 extends between paddles 120and 121 and has a first base surface 115 a continuous with base 112 a oftrack 112, a second base surface 115 b continuous with base 113 a oftrack 113 and a tapered distal tip 115 c coterminus with tapered distalends 120 c and 121 c. Paddles 120 and 121 also have a width dimensionW_(P) therebetween, spaced apart by a width of spacing member 115.Tapered distal tips 115 c, 120 c and 121 c facilitate insertion of thepaddles between adjacent bones.

Paddle 120 has a major height dimension P_(H1) between edge 120 a and120 b. Paddle 120 also has a minor height dimension P_(H2) between basesurface 115 a and edge 120 a and an equal minor height dimension P_(H2)between base surface 115 b and edge 120 b. Paddle 121 has the sameheight dimensions P_(H1) and P_(H2) as paddle 120.

In the illustrated embodiment, a portion of track 112 includes a wall125 extending between rails 112 b and 112 c and parallel to base 112 a.Shown best in FIG. 10, wall 125 extending between rails 112 b and 112 cforms an enclosed lumen 126 over a proximal portion of track 112. In asimilar manner, a portion of track 113 includes a wall 127 extendingbetween rails 113 b and 113 c which forms enclosed lumen 128 in thatportion of track 113 where wall 127 is present. Each of lumens 126 and128 has a height dimension L_(H). As best seen in FIG. 10, if walls 125and 127 are ignored, and channel guide 103 viewed from the proximal endwith rail spacing member 114 oriented in a vertical plane, channel guide103 can have an “I beam” shaped configuration.

At the junction of the distal end 110 of channel guide 103 with paddles120 and 121, shoulders 130 a, 130 b and 131 a, 131 b are formed.Shoulders 130 a-131 b provide an affirmative stop to stop distaladvancement of bone cutting instrument 100 when paddles 120 and 121 areinserted into an intervertebral disc space between adjacent vertebrae.

FIGS. 11-14 illustrate an alternative embodiment of a channel guide 150according to the invention. Channel guide 150 is substantially identicalto channel guide 103 except that channel guide 150 has a circularcross-section. However, similar to channel guide 103, channel guide 150includes a first track 151 and a second track 152. Track 151 includes abase 151 a, a first rail 151 b and a second rail 151 c. Likewise, track152 includes a base 152 a, a first rail 152 b and a second rail 152 c.Extending a portion of the length of channel guide 150 from proximal end153, rails 151 b and 151 c are continuous with one another forming anenclosed lumen 155 over track 151. A similar enclosed lumen 156 ispresent over track 152. Each of hemi-circular lumens 155 and 156 has amaximum lumen height L_(H).

Paddles 157 and 158 extend distally from the distal end 160 of tracks151 and 152 respectively. Paddles 157 and 158 have a curvedcross-section and each has a major height dimension P_(H1) extendingfrom edge 157 a to edge 157 b and from edge 158 a to edge 158 b. Each ofpaddles 157 and 158 have a first and second minor height dimensionP_(H2) as described for channel guide 103. Shoulders 170 a 170 b areformed at the junction of distal end 160 and paddles 157 and shoulders171 a and 171 b are formed at the junction of distal end 160 and paddle158. It will be appreciated that various cross-sectional configurationsfor channel guides are within the scope of the invention, in addition tothe rectangular cross-section of channel guide 103 and the circularcross-section of channel guide 150.

FIG. 15 is a top plan view of mandrel 104 (105 being identical) shown inFIG. 5; FIG. 16 is side plan view of mandrel 104 and FIG. 17 is atransverse cross-section view of mandrel 104 taken through line 16—16.Mandrel 104 includes a distal end 201, a proximal end 202 and a shaft203 extending therebetween. As best seen in FIG. 15, mandrel 104 has agap surface 205 and a tapered distal tip 206 at distal end 201. Mandrel104 has a shaft height M_(S) along a portion of shaft 203, a distal endheight M_(D) at distal end 201 and a proximal end height M_(P) atproximal end 202. Preferably, distal end height M_(D) is substantiallyequal to minor height P_(H2) of paddles 120 and 121. Thus, when heightM_(D) of mandrel 104 is equal to minor height P_(H2) of paddles 120 and121, a flush surface is provided extending from edge 120 a of paddle 120across gap surface 205 and edge 121 a of paddle 121 (see FIG. 5). Asimilar flush surface is formed between mandrel 105 and paddle edges 120b and 121 b.

With mandrels 104 and 105 inserted within tracks 112 and 113 the spacebetween paddles 120 and 121 is filled out. Thus, when inserted into anintervertebral disc space, pressure exerted by the bone cuttinginstrument 100 on each of the opposing vertebrae is not localized onlyon the edges of the paddles, but rather the pressure is distributedacross the entire surface area between the paddles and including the gapsurfaces of the mandrels. It will be appreciated that if the distractingguide has a cylindrical cross-section as illustrated in, for example,FIG. 11, the mandrel will have a corresponding shape including thefeatures described for rectangular shaped mandrel 104 and 105.

Shaft height M_(S) of mandrel 104 is provided to pass within trackheight L_(H) in close tolerance within lumen 126 (or 128) of channelguide 103. The proximal end height M_(P) of mandrel 105 at the proximalend 202 is selected to be greater than shaft height M_(S) of shaft 203to form a shoulder 207. Shoulder 207 affirmatively stops distaladvancement of mandrel 104 along track 112 when shoulder 207 abuts railspacing member 114 of channel guide 103. Second mandrel 105 can beconfigured identical to mandrel 104 to pass into track 113 and from basesurface 115 b to edges 120 b and 121 b.

In a typical embodiment, paddle major height dimension P_(H1) can beabout 3 to 15 mm, paddle minor height dimension P_(H2) about 1 to 7 mm,lumen height dimension L_(H) about 2 to 13 mm and mandrel proximalheight dimension M_(P) of about 1 to 2 mm greater than lumen heightdimension L_(H.) For example, in the illustrated embodiment 100, thepaddle major height dimension can be P_(H1) is 8 mm, the paddle minordimension P_(H2) can be about 3.5 mm, the lumen height dimension L_(H) 5mm and the mandrel proximal height dimension about M_(P) 6 mm.

FIG. 18 is a top plan view of one embodiment of a bone chisel 106 shownin FIG. 6. FIG. 19 is a longitudinal cross-section view through line19—19, and FIG. 20 is a distal end-on view of bone chisel 106. Bonechisel 106 includes a proximal end 301, distal end 302 and shaft 303therebetween. Cutting surface 304 is at distal end 302. Cutting surface304 is a rectangular cutting surface 305 including longitudinal cuttingedge 306 and first lateral cutting edge 307 and second lateral cuttingedge 308.

Distal end 302 of chisel 106 has a first chisel height C₁. In a typicalembodiment, the difference between first chisel height C₁ and paddleminor dimension P_(H2) determines the amount of bone removed from a boneend during bone cutting. Thus, to remove about 1 mm of bone from the endof the bone, the difference between C₁ and P_(H2) is about 1 mm. C₁ istypically selected to be about 1 to 3 mm greater than paddle minordimension P_(H2). In the case of cutting bone from vertebral endplates,C₁ is preferably sufficient to cut deep enough into the endplate toremove the articular cartilage and cortical bone to expose cancellousbone.

The distal end 302 of bone chisel 106 also includes a groove 310extending a distance proximally from cutting surface 304 between cuttingedges 306-308. As illustrated in the longitudinal cross-section view ofFIG. 19, groove 310 has a depth less than chisel height C₁ and providesfor proximal passage of bone, cartilage or other debris as chisel 106 isadvanced distally to cut between adjacent bones.

At proximal end 301, bone chisel 106 has a second chisel height C₂. Ashoulder 320 is formed where chisel heights C₁ and C₂ meet. Chiselheight C₁ is selected to provide for bone chisel 106 to pass in closetolerance within lumen 126 (or 128) of channel guide 103. Shoulder 320affirmatively stops distal advancement of bone chisel 106 within tracks112 or 113 when shoulder 320 abuts against wall 125 or 127 at theproximal end 111 of channel guide 103.

FIGS. 22-49 disclose various components of an embodiment for implantingan implant (e.g., implant 900 of FIG. 49) into an intervertebral space.Generally, the system includes a set of differently sized rasps (e.g.,see rasp 600 of FIGS. 25 and 26) for determining the implant sizerequired to match an intervertebral space. The system also includes aset of differently sized cutting tools (e.g., see cutting instrument 510of FIGS. 22-24) for cutting bone surrounding the intervertebral space.The size and shape of the cutting tool selected is determined by thesize and shape of the rasp. Preferably, the cutting tool slides onto therasp. Before or after cutting, the rasp can be manipulated to conditionor roughen a portion of the site for the implant. Thereafter, the raspand the cutting tool can be removed. Finally, the implant(s) is insertedinto the space.

FIG. 22 is a top view and FIG. 23 a side view of another alternativeembodiment of a bone cutting instrument 510 according to the invention.Instrument 510 has a proximal end 515 and a distal end 516 spaced alonglongitudinal axis X—X. At the proximal end 515 of shaft 517 there is ahandle 518 for operating instrument 510. The handle 518 has a roughenedarea 519 that can be in the form of knurls, etchings, grooves, ridges,or other suitable patterns to enhance manual gripping of the handle 518.At the distal end 516, instrument 510 includes a first cutting edge 520,a second cutting edge 521, and third and fourth cutting edges 522 and523. In the illustrated embodiment, cutting edges 520, 521, 522 and 523are at the distal end of chamber 525. First, second, third, and fourthcutting edges 520, 521, 522 and 523 are beveled 520 a, 521 a, 522 a, and523 a, respectively, to facilitate cutting and removal of bone. Aninternal hollow bore 527 extends from the proximal end 515 through theinstrument 510 to the distal end 516 to receive a rasp 600 and toreceive bone.

In the illustrated embodiment, elongated openings 550 and 551 extendthrough the handle 518 and shaft 517, respectively, of the instrument510. As described later in the specification, opening 550 allows foralignment of the cutting instrument 510 with rasp 600. Opening 551provides additional access to the internal bore 527 for cleaning theinstrument and reduces the weight of the instrument.

FIG. 24 is a distal end-on view of instrument 510 showing that first andsecond cutting edges 520 and 521 define a height dimension C_(H) and thecutting edges 522 and 523 define a width dimension W_(C). The perimeterconfiguration of cutting edges 520, 521, 522, and 523 in FIG. 24 is arectangular shape particularly suited for preparing a channel or implantbore between adjacent bones for insertion of a two-part implant having aconfiguration such as that of the implant 900 shown in FIG. 49.

In the illustrated embodiment of FIG. 49, implant 900 is shown withgrowth component 901, such as cancellous bone, and support component902, such as cortical bone. The growth component 901 has a similar sizeand shape as the distal end of the cutting instrument 510 (e.g.,dimension A of growth component 901 corresponds to dimension W_(C) ofcutting instrument 510 and dimension B of growth component 901corresponds to dimension C_(H) of cutting instrument 510). Also, therounded nose of the growth component 901 corresponds to the curvature ofedges 520 and 521 of the cutting instrument 510. The support component902 has a similar size and configuration as the rasp head (see forexample FIGS. 25, 26). The support component 902 of the implant may bethe same size as the rasp head, or it can be larger than the rasp head.The support component 902 of the implant can be about 0 mm to about 4 mmlarger in height than the rasp head. The height dimension C_(H) of thebone cutting instrument can be from about 0 mm to about 10 mm talleroverall than the support component of the implant. It will beappreciated, however, that the perimeter configuration of cutting edges520, 521, 522, and 523 can be square, circular, oval, etc., depending onthe external configuration of the implant to be inserted into thechannel. The length of the first and second cutting edges 520 and 521can vary to correspond with the depth of the vertebrae.

To cut different sized channels, a set of bone cutting instruments 510will be available which has instruments with incrementally differentsizes of cutting edges 520, 521, 522, 523 corresponding to a particularsize implant. For example, bone cutting instruments 510 having first andsecond cutting edges 520, 521 with different heights C_(H) will beavailable to permit the surgeon to select a cutting edge heightcorresponding to a particular disc space height. In addition, it will beappreciated that the illustrated cutting edges 520 and 521 (and 522 and523) are parallel. In alternative embodiments, cutting edges 520 and 521(and 522 and 523) can form a converging or diverging taper.

FIG. 25 is a top view and FIG. 26 a side view of a rasp 600 according toone embodiment of the invention. The rasp 600 is inserted into theintervertebral space, and functions both as a trial sizer, i.e. for aparticularly sized and shaped implant, and rasp. Rasp 600 has a proximalend 601 and a distal end 602 spaced along longitudinal axis X—X. At theproximal end 601 of shaft 603 there is a roughened area 604 that can bein the form of knurls, etchings, grooves, ridges, or other suitablepatterns to enhance manual gripping of the shaft 603. An opening 605 forreceiving a portion of an alignment pin extends transversely through theproximal end 601 of the shaft 603, and allows for alignment of the raspwith cutting instrument 510.

At the distal end 602, rasp 600 includes a rasp head 606. In theillustrated embodiment, rasp head 606 includes an outer wall 607, aninner wall 608 and has a generally “C-shaped” configuration with a firstarm 609 continuous with a second arm 610. The inner wall 608 defines apocket or receptacle which is sized to complement and receive the distalend of the cutting instrument 510. The first arm 609 and second arm 610are spaced apart from the shaft 603. Rasp head 606 includes a firstengaging surface 611 and a second engaging surface 612. In theillustrated embodiment, the first and second engaging surfaces 611, 612have ridges 613.

As illustrated best in FIG. 27, in this embodiment, rasp head 606 has amajor height H_(M) and minor height H_(m). The taper from the majorheight to the minor height can be from about 0° to about 16°. The shapeand configuration of the rasp head 606 corresponds to the shape andconfiguration of an implant. In one embodiment, the rasp head 606corresponds in size and configuration with the support component 902 ofa two-part implant 900. The space between the first and second arms 609,610 of the rasp head 606 corresponds with the growth component 901 ofthe implant. It will be appreciated, however, that the configuration ofthe rasp head 606 can be square, rectangular, circular, oval, etc.,depending on the configuration of the implant(s) to be inserted into thechannel.

As a trial sizer, the rasp 600 provides a means for determining theappropriate size bone cutting instrument and implant to use for aparticular implant site. Multiple rasps 600 are provided, withincrementally different sized, shaped, and/or tapered rasp heads 606corresponding to different sized, shaped, and/or tapered implants. Thesurgeon inserts and removes the various rasps 600 and determines whichone is the correct size for the intervertebral space. The ridges 613 onthe upper and lower surfaces of the rasp head act as a rasp to conditionthe upper and lower surfaces of the channel between the vertebrae.

Proximal to the distal end 602, the shaft 603 of the rasp 600 alsoincludes markings 614 at predetermined distances from the distal edge615 of the rasp head. During use, markings 614 provide the surgeon withan indication of the depth of distal penetration of rasp 600 betweenadjacent bones.

In one embodiment, the rasp shaft 603 is slidably received within theinternal bore 527 of the bone cutting instrument 510, with the opening605 in the rasp shaft 603 accessible through the opening 550 in the bonecutting instrument handle 518. An alignment pin 700 (see FIGS. 30-39) isinserted through openings 550, 605 to align the rasp within the bonecutting instrument. When the alignment pin 700 is in place, the shaft603 of the rasp rides within the opening 550 in the bone cuttinginstrument 510 to maintain rotational alignment and limit the axialmovement to within a predetermined range corresponding to the length ofopening 550. The alignment pin 700 also can function as a handle whenthe bone cutting instrument/rasp combination is forced between (e.g.hammered into) the adjacent bones to create the implant channel.

The alignment pin 700 has a handle 701, a collar 702, and a pin 703. Thehandle 701 can have a roughened surface that can be in the form ofknurls, etchings, grooves, ridges, or other suitable patterns to enhancemanual gripping of the handle 701. Alternatively, the handle 701 can bea shape that enhances manual gripping. In the illustrated embodiment,the handle 701 has indentations 704 to enhance manual gripping. Thecollar 702 is mounted to the handle 701 and the pin 703 is mounted tothe collar 702. The collar 702 can be of any shape. In the illustratedembodiment, the collar 702 has a circumferential ridge 705. The ridge705 has opposing flattened areas 706 that allow for unimpeded hammeringof the bone cutting instrument and rasp while manually grasping thealignment pin.

Once a channel is cut into adjacent bones, an implant or implants areinserted to fuse the adjacent bones and provide stability to the site.In one embodiment, a two part implant 900 such as that shown in FIG. 49is used. The support component 902 is inserted into the channel, andthen the growth component 901 is inserted. FIGS. 40 and 41 illustrateone embodiment of an implant insertion tool 800 suitable for use withembodiments of the bone cutting instrument 510 and rasp 600 of theinvention. As illustrated, implant insertion tool 800 has a proximal end801 and a distal end 802 having a working end 803. Working end 803includes tabs 804 and 805 that fit cooperatively within grooves 903 ofthe support component 902 of an implant (see FIG. 49). In addition, theworking end 803 includes a slot 806 that permits resilient/elastic arms807 and 808 to flex or expand laterally away from axis A_(T).

In a typical embodiment, arms 807 and 808 are spring biased to expandaway (e.g., laterally) from axis A_(T) in the normal, relaxed position.A sleeve 820 (FIGS. 43-45) can then be slid from the proximal end 801 ofthe insertion tool 800, over the slot 806, to force arms 807 and 808towards (e.g. medially) axis A_(T). That is, when the sleeve is advanceddistally it brings arms 807 and 808 together towards axis A_(T). In thisposition, the working end 803 of implant insertion tool 800 can beinserted into an implant. Similarly, where useful for additionalcontrol, tabs 804 and 805 can be inserted into grooves 903 of animplant. The sleeve can then be slid towards the proximal end to allowarms 807 and 808 to expand away from axis A_(T) to provide frictionholding of an implant on the working end 803. After placement of animplant, the sleeve can be slid distally to bring arms 807 and 808 backtoward axis A_(T) to remove implant insertion tool 800, leaving theimplant in place. Other arrangements providing for expansion andcontraction of arms 807, 808, relative to axis A_(T) also arecontemplated by this disclosure

Thus, an implant can be mounted on the working end 803 of implantinsertion tool 800 allowing the surgeon to manipulate an implant viatool 800 into a suitable position at the fusion site.

In one embodiment, the implant insertion tool 800 includes a sleeve 820(FIGS. 43-45) and a handle 850 (FIGS. 46-48). In the illustratedembodiment, the insertion tool 800 has a threaded region 809 at theproximal end 801. The threaded region 809 mates with the distal end 851of the handle 850. The handle has a roughened area 852 that can be inthe form of knurls, etchings, grooves, ridges, or other suitablepatterns to enhance manual gripping of the handle 850. In oneembodiment, the distal end 851 of the handle 850 has exterior threadingto match internal threading 821 on a sleeve 820.

The sleeve 820 is hollow and has a bore 822 extending from the proximalend 823 to the distal end 824, and which is sized to fit over theproximal end 801 of the implant insertion tool 800, and to threadiblyadvance, i.e. distally, over slot 806, to force arms 807 and 808 towardsaxis A_(T).

II. Methods

The instruments of the invention can be used to prepare a channel of aselected configuration between adjacent bones, and to insert an implantor implants into the prepared channel. For exemplary purposes, themethods of the invention will be described with respect to preparing achannel between adjacent vertebral bodies. However, it will beappreciated that the principles and methods can also be applied topreparing a channel between other bones.

The present invention will first be described with reference to use in aposterior approach. In a posterior approach, a surgeon seeks access tothe spine through the back of the patient. An alternative approach isthe lateral approach where the patient is on his side. Anotheralternative approach is an anterior approach where the surgeon seeksaccess to the spine through the abdomen of a patient. The approaches canbe done through an open or laparoscopic procedure.

With reference to FIG. 21, once a surgeon has identified two vertebraethat are to be fused, e.g., lumbar vertebrae V₁ and V₂, the surgeondetermines the size of the desired implant and the desired amount ofdistraction of the intervertebral disc space IVS required beforeplacement of the implant.

Exposure of the intervertebral disc can be obtained through any suitabletechnique known in the art. Preferably, the facet of the vertebrae isremoved in as limited amount as possible to permit insertion of theimplant site preparation instruments and the implant. Single or multipleimplants can be used. If a single implant is used, the implant istypically positioned centrally within the lateral margins of the discspace. If a pair of implants is used, they are positioned on either sideof the midline of the vertebrae and within the lateral margins of thedisc space. If a single implant is used, the transverse (width)dimension of the implant will generally be greater than the transversedimension of a single one of a pair of implants. A single implant ismore likely to be used in a lateral or anterior approach than aposterior approach due to restrictions on the amount of lateralretraction that can be applied to the spinal cord.

Continuing with the posterior approach to lumbar vertebrae V₁ and V₂,after lateral retraction of the cauda equina, a partial or fulldiscectomy can be performed using known methods, being careful tomaintain as much of the annulus as possible. A bone cutting instrument100 (including mandrels 104, 105) having paddles with a major heightdimension P_(H1) approximating the desired disc space height is passedinto a first side of the intervertebral disc space between adjacentvertebrae V₁ and V₂. In one embodiment, a distraction spacer, such asshown in FIG. 28 of U.S. Pat. No. 5,489,307, or similar device, can beused to maintain distraction of the disc space on a second side (i.e.,contralateral to the first side being prepared) of the vertebral bodiesV₁ and V₂. If a distraction spacer is used, after preparation of thefirst side, the implant can be inserted into the channel prior topreparation of the channel on the second side. Alternatively, afterpreparing the channel on the first side, the bone cutting instrument canbe removed and the cauda equina retracted over the first side and thechannel on the second side prepared before inserting the implants.

During insertion of the paddles 120, 121, of bone cutting instrument 100(FIG. 5), it may be advantageous to initially insert a paddle having asmaller than desired paddle height dimension P_(H1) and sequentiallyinsert instruments having increasing paddle heights P_(H1) until thedesired disc space height is achieved. Once the tapered distal ends ofthe paddles are inserted into the disc space, the proximal ends ofchannel guide 103 and mandrels 104 and 105 can be tapped (i.e.,typically as a single unit), for example, with an orthopedic hammer, toadvance the paddles into the disc space until the shoulders 130, 131abut the posterior surfaces of the vertebral bodies.

The first mandrel 104 can then be removed and replaced with bone chisel106 that is passed along track 112. The proximal end 301 of chisel 106is then tapped into first vertebrae V₁ to cut a first notch 400 intoendplate 401. Chisel 106 is then removed and can be replaced by firstmandrel 104. Second mandrel 105 can then be removed and replaced withbone chisel 106 that is passed along track 113 into second vertebrae V₂to cut a first notch 402 into endplate 403 of second vertebrae V₂. Thebone channel guide 103 with mandrel 104 and bone chisel 106 can then beremoved leaving channel 404 defined by notches 400 and 402 as indicatedwith broken lines in FIG. 21. An implant, such as a rectangular boneplug can then be inserted into channel 404 on the first side and theabove procedure repeated on the second side. If the bone cuttinginstrument has a circular cutting edge, and a threaded implant is to beused, the channel formed can be threadedly tapped using known tappinginstrumentation.

In an alternative embodiment, after lateral retraction of the caudaequina and discectomy, a bone cutting instrument 10 (FIGS. 1-3) having apreselected paddle height P_(H) can be inserted into the first side ofthe intervertebral disc space. As described above, cutting instruments10 having paddles of increasingly greater paddle height dimension P_(H)can be sequentially inserted into the disc space until the appropriatedisc height is established. A distraction spacer may be used on thecontralateral side as previously described. After the tapered distalends 20 a, 21 a of paddles 20 and 21 having the appropriate heightdimension P_(H) are inserted into the intervertebral disc space (IVS),the bone cutting instrument 10 is advanced until bone cutting surface 23contacts the posterior surfaces of vertebrae V₁ and V₂. At this point,without removing the bone cutting instrument, the proximal end 15 ofinstrument 10 can be tapped to further advance cutting edge 23 tosimultaneously remove bone from the endplates of the adjacent vertebraeV₁ and V₂. Bone and disc material cut by cutting surface 23 will passinto chamber 25 and out opening 24. Advancement of cutting instrument 10into the intervertebral disc space is continued until the paddles (orcutting edge) have reached a predetermined depth that can be indicatedby marks 30. Alternatively, the depth to which cutting instrument 10 isinserted into the disc space can be determined by visualization methodssuch as x-ray, MRI, CT scan, etc.

Once the appropriate depth has been reached, bone cutting instrument 10is removed and any debris remaining in the channel can be removed usinga rongeur, osteotome, forceps, etc. If a threaded implant is to be used,the channel formed by cutting instrument 10 can be tapped using knownmethods for tapping an implant bore. If a second implant site is to beprepared, the first implant can be inserted prior to preparation of thesecond implant site or both implants inserted after both implant sitesare prepared.

Again with reference to FIG. 21, once a surgeon has identified twovertebrae that are to be fused, e.g., lumbar vertebrae V₁ and V₂, thesurgeon can use the rasp to determine the size of the desired implantand the desired amount of distraction of the intervertebral disc space(IVS) required before placement of the implant. Exposure of theintervertebral disc can be obtained through any suitable technique knownin the art, such as by using a distractor. The distractor is insertedbetween the adjacent bone surfaces, and a rasp is passed through theadjacent bone surfaces to space the bone surfaces apart a predetermineddistance. The distractor provides an exposure window through which thebone cutting instrument, rasp, and implant inserting tool with theimplant can be inserted, as is more thoroughly described in U.S. Pat.No. 6,224,599 to Baynham, which is incorporated herein by reference.

The surgeon can determine the size of the desired implant 900 and canselect the appropriately sized bone cutting instrument 510, and implantinserting tool 800 using a series of rasps 600. The rasp 600 functionsas a trial sizer in that the rasp heads 606 correspond to variousincremental implant sizes and shapes. The dimensions of the rasp head606 are proportional to the dimensions of the bone cutting instrument510, implant inserting tool 800 and the implant 900. Once a rasp 600 isfound that corresponds to the size of the intervertebral space for aparticular patient between particular vertebrae, the rasp 600 can bemoved into and out of the space to prepare the vertebrae for receivingthe implant 900. The roughened surfaces 611, 612 on the rasp head 606function as a rasp to provide a more uniform andosteoconductive/osteoinductive site for the implant.

Leaving the rasp 600 in place, the surgeon selects the appropriatelysized bone cutting instrument 510 and slides it over the rasp shaft 603,aligning the openings 550, 605 in the proximal shafts of the bonecutting instrument and rasp. The alignment pin 700 is inserted into theopenings to maintain the alignment, and the bone cutting instrument 510is forced, e.g. forced, into place. The openings 550, 605 in the shaftsof the bone cutting instrument and rasp provide a stop when aligned withthe alignment pin 700 to prevent the bone cutting instrument 510 fromcutting too deep into the intervertebral space. The inner surface 608 ofthe rasp head 606 also acts as a stop for the third and fourth cuttingedges 522, 523 of the bone cutting instrument 510.

Once the channel has been cut, a slap hammer can be attached to theshaft 517 of the bone cutting instrument 510 to remove the instrumentand the rasp 600. The appropriately sized support component 902 of theimplant 900 is positioned on the insertion tool 800 with the sleeve 820in the proximal position, and the implant is inserted into the preparedchannel. The sleeve 820 is then advanced distally to force the arms 807,808 of the insertion tool toward one another to release the implant. Ifa two-part implant is used, the second part of the implant (growthcomponent 901) is then inserted.

From the foregoing detailed description and exemplary embodiments, itwill be evident that modifications and variations can be made in thedevices and methods of the invention without departing from the spiritor scope of the invention. Therefore, it is intended that allmodifications and variations not departing from the spirit of theinvention come within the scope of the claims appended hereto.

What is claimed is:
 1. A bone cutting instrument comprising: a proximalend and a distal end spaced apart along a longitudinal axis of theinstrument; first and second cutting edges at the distal end, the firstand second cutting edges being diametrically opposed; and third andfourth cutting edges at the distal end, the third and fourth cuttingedges being diametrically opposed and adjacent the first and secondcutting edges; wherein the first, second, third, and fourth cuttingedges define an interior void, and the first and second cutting edgesextend distally beyond the third and fourth cutting edges; wherein thefirst and second cutting edges are radiused.
 2. The bone cuttinginstrument according to claim 1 wherein a relationship of the first,second, third, and fourth cutting edges defines a parallelogram.
 3. Thebone cutting instrument according to claim 1 further comprising a shaftdisposed between the proximal and distal ends, the shaft comprising anelongated opening at the proximal end, substantially perpendicular tothe longitudinal axis.
 4. The bone cutting instrument according to claim3 further comprising a hollow bore extending from the proximal end tothe distal end.
 5. The bone cutting instrument of claim 1 furtherincluding indicator markings to determine depth of cutting.
 6. A bonecutting instrument comprising: a proximal end and a distal end spacedapart along a longitudinal axis of the instrument; a hollow boreextending from the proximal end to the distal end; first and secondcutting edges at the distal end, the first and second cutting edgesbeing diametrically opposed and radiused; and third and fourth cuttingedges at the distal end, the third and fourth cutting edges beingdiametrically opposed and adjacent the first and second cutting edges;wherein the first, second, third, and fourth cutting edges define aninterior void; wherein a relationship of the first, second, third, andfourth cutting edges defines a parallelogram, and the first and secondcutting edges extend distally beyond the third and fourth cutting edges.7. A bone cutting instrument comprising: a proximal end and a distal endspaced apart along a longitudinal axis of the instrument; first andsecond cutting edges at the distal end, the first and second cuttingedges being diametrically opposed; and third and fourth cutting edges atthe distal end, the third and fourth cutting edges being diametricallyopposed and adjacent the first and second cutting edges; wherein thefirst second, third, and fourth cutting edges define an interior void,wherein the first and second cutting edges have a convex curvatureextending between the third and fourth cutting edges, and the first andsecond cutting edges extend distally beyond the third and fourth cuttingedges.
 8. The bone cutting instrument of claim 7 further comprising ahollow bore extending from the proximal end to the distal end.
 9. Thebone cutting instrument of claim 7 wherein a relationship of the first,second, third, and fourth cutting edges defines a parallelogram.